In-plane microwave penetration depth ${\lambda }_{ab}$ and quasiparticle conductivity at 28 GHz are measured in underdoped single crystals of the Fe-based superconductor ${\mathrm{PrFeAsO}}_{1-y}$ ($T_c\approx 35\mathrm{K}$) by using a sensitive superconducting cavity resonator. ${\lambda }_{ab}(T)$ shows flat dependence at low temperatures, which is incompatible with the presence of nodes in the superconducting gap $\Delta (\mathbf{k})$. The temperature dependence of the superfluid density demonstrates that the gap is nonzero ($\Delta /k_B{T}_c\gtrsim 1.6$) all over the Fermi surface. The microwave conductivity below $T_c$ exhibits an enhancement larger than the coherence peak, reminiscent of high-$T_c$ cuprate superconductors.

4SEASONS is a Fermi chopper spectrometer in operation at the Materials and Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC). 4SEASONS is expected to facilitate high-efficiency measurements of weak inelastic signals on novel spin and lattice dynamics using thermal neutrons. The spectrometer is equipped with a coupled moderator and sophisticated components such as an elliptical converging neutron guide with high-critical-angle supermirrors, long (2.5 m) position-sensitive detectors, and a Fermi chopper appropriate for multiple-incident-energy (multi- E i ) measurements by the repetition-rate multiplication technique. Herein, we discuss in detail the design and performance of the spectrometer, and present some examples of the measurements obtained using this spectrometer.

Giant negative thermal expansion is achieved in antiperovskite manganese nitrides when the sharp volume change associated with magnetic ordering is broadened by substitution. In this Letter, we address the unique role of the ‘‘magic” element, Ge, for such broadening in ${\mathrm{Mn}}_3{\mathrm{Cu}}_{1-x}{\mathrm{Ge}}_x\mathrm{N}$. We present evidence for a local lattice distortion well described by the low-temperature tetragonal ($T_4$) structure of ${\mathrm{Mn}}_3\mathrm{GeN}$ for a range of $x$, where the overall structure remains cubic. This structural instability shows a strong correlation with the broadness of the growth of the ordered magnetic moment and, hence, is considered to trigger the broadening of the volume change.

We report a Fe K$\beta$ x-ray emission spectroscopy study of local magnetic moments in various iron-based superconductors in their paramagnetic phases. Local magnetic moments are found in all samples studied: PrFeAsO, $\mathrm{Ba}{(\mathrm{Fe},\mathrm{Co})}_2{\mathrm{As}}_2$, LiFeAs, Fe${}_{1+x}$(Te,Se), and ${\mathrm{A}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ (where $\text{A}=\text{K}$, Rb, and Cs). The moment size is independent of temperature or carrier concentration but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1${\mu }_B$/Fe, while FeTe and ${\mathrm{K}}_2{\mathrm{Fe}}_4{\mathrm{Se}}_5$ families have much larger local moments of $\sim 2{\mu }_B$/Fe and $\sim 3.3{\mu }_B$/Fe, respectively. Our results illustrate the importance of multiorbital physics in describing magnetism of these compounds.